1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a digital copying machine, a laser beam printer, a laser beam plotter, a facsimile machine, etc.
2. Discussion of the Background
In an electrophotographic image forming apparatus such as a digital copying machine, a laser beam printer, a laser beam plotter, a facsimile machine, etc., an image is formed by at least the following processes. First, a surface of an image bearing member such as a photoreceptor is scanned by a laser beam according to image data, and thereby a latent image is formed on the image bearing member. The latent image is developed with toner stored in a developing device. Subsequently, a toner image is transferred onto a transfer material such as a recording sheet. Thereafter, an unfixed toner image is fixed onto the transfer material.
A background image forming apparatus described in Japanese Laid-open patent publication No. 9-197776 has an objective of performing image forming processes with accuracy regardless of differences between apparatuses and environmental conditions. In order to achieve this objective, in an image forming apparatus employing an overlapped scanning exposure method, a plurality of exposure patterns are formed on a photoreceptor to be developed with toner into toner images. An average toner density measuring device measures the average toner density of each toner image. A relationship between a quantity of light of each laser beam forming the exposure patterns and an image edge position is obtained from the measured average toner density, and is stored in a memory of an adjusting device. The image forming processes are performed with accuracy regardless of differences between apparatus and environments by controlling the quantity of light of each laser beam based on the relationship between the quantity of light of each laser beam forming the exposure patterns and the image edge position.
Another background image forming apparatus described in Japanese Laid-open patent publication No. 10-181091 has an objective of precisely forming images having twice the resolution of an image writing device including an optical scanning system. In order to achieve this objective, the image forming apparatus includes an image data source that supplies image data having a resolution twice that of an optical scanning device. An image pattern judging device judges the pattern in a sub-scanning direction of the image data input from the image data source. An exposure control device outputs a laser beam lighting signal and a laser beam intensity signal for reproducing the image data according to the image data by an optical scanning device having a resolving power half that of the image data. A modulation exposure device modulates the laser beam based on the laser beam lighting signal and laser beam intensity signal from the exposure control device, and a photoreceptor is scanned by the laser beam modulated by the modulation exposure device.
When a latent image writing device, including an optical scanning system or a plurality of optical scanning systems, emits a plurality of laser beams, and when images are formed at twice the resolution of the latent image writing device, the following conditions (1)-(2)are required to precisely form images. (1) A density of a toner image formed by any one of the plurality of laser beams should be equal each other. (2) A density of a toner image formed by any two adjacent partially overlapped such beams should be equal to that of each toner image formed by any one of the plurality of laser beams.
Generally, a quantity of light of each laser beam forming latent images on an image bearing member and an image writing time are preset for an image forming apparatus. However, the density of toner images varies between apparatuses, depending on environmental conditions and the lapse of time. Therefore, in order to accurately reproduce images according to image data input to the image forming apparatus, the quantity of light of each laser beam is desired to be adequately adjusted.
According to one aspect of the present invention, an image forming apparatus includes an image bearing member, and a latent image writing device a mechanism to emit a plurality of laser beams, two adjacent of the plurality of laser beams partially overlapping in a sub-scanning direction, respectively, and a latent image writing device configured to emit a plurality of laser beams, and a mechanism to form a latent image of a reference image on a surface of the image bearing member by performing repeated scanning of each laser beam forming the latent image of the reference image on the surface of the image bearing member. The latent image writing device has a plurality of modes of writing the latent image of the reference image.
The image forming apparatus further includes a developing device configured to develop the latent image of the reference image formed on the surface of the image bearing member with toner into a toner image of the reference image, and a detecting device configured to detect the density of the toner image of the reference image on the image bearing member. The quantity of light of each laser beam forming a subsequent latent image is adjusted according to the density of the toner image of the reference image detected by the detecting device.